Treat IJ SDK and plugins as managed dependencies

[mutcianm]

At the moment all IntelliJ SDK and plugin jars are dumped into unmanagedJars. This creates a lot of inconveniences, e.g. :

• jar roots are duplicated per each subproject
• no separation of IJ platform jars and plugin jars
• no separation of different plugin jars
• no version information for the platform/plugin jars